WASHINGTON -- The Supreme Court says states cannot require would-be voters to prove they are U.S. citizens before using a federal registration system designed to make signing up easier.

The justices voted 7-2 to throw out Arizona's voter-approved requirement that prospective voters document their U.S. citizenship in order to use a registration form produced under the federal "Motor Voter" voter registration law.

The 9th U.S. Circuit Court of Appeals said that the 1993 National Voter Registration Act, which doesn't require such documentation, trumps Arizona's Proposition 200 passed in 2004. Arizona officials say their law is needed to stop non-Americans from voting in elections, while opponents see it as an attack on minorities, immigrants and the elderly.